I have a group with 4 mages and run them all scorch spec (including imp scorch).

I don't use fireball very much, and I didn't take imp fireball. Reason is, with 4 toons you have a 40% chance on each round of casts for an impact proc, which is darn sweet. Most of the times, mobs don't even get to me because of a stun so I take no dmg.

Can't say for sure with less than 4, but just thought I'd throw that out there.